Trying to parallel 4 smoke units off one board would be 1 ohm effective and surely damage trace or fet. Stan does show the heater FET in his picture.
Wouldn't 4 parallel be 2 ohms? In any event with say 20V PV driving 2 ohms that would be 10 Amp pulses which does seem a bit aggressive. 
I would be careful loading up the 5V side. Not necessarily the FET, but think of the load on the 1 amp 5V power supply, that is meant to run the micro chips and power the Processor Power Supply. That is why a bad fan motor can cause a PS-2 board to miss behave. G
The question is whether the slave board has its own 5V DC supply that powers the "high side" voltage to the slave's fan motor(s). It looks like a relatively un-cluttered board from a layout perspective so it should be easy to visually trace the fan power source from the 4-pin smoke connector. If it goes to a local 5V supply (instead of coming from the tether) then it should not load the master's 5V supply. Then the issue is whether the master's fan FET is capable of driving 4 motors...in my opinion of course.
